There is certain problem arising when daily statistics have many segments and/or many idsites. In file https://github.com/piwik/piwik/blob/master/core/DataAccess/ArchiveSelector.php#L317

there ocassionaly is being triggered huge DELETE query with up to 20k elements in "WHERE IN" clause. This causes server to drop connection to DB server.

Maybe it could be fixed by introducing new config variable allowing to define sharding limit for this query. For ex. it could be [General] purging_outdated_archives_limit = 0 . By default it would be set to 0 - then no sharding will be applied (for small/medium instances where this does not cause problems). After changing to bigger value - single query should be replaced with iteration of queries with X idarchives in one batch executed for all necessary idarchives.